Titanium-vanadium-tin comprising catalyst and process for the preparation of phthalic anhydride

ABSTRACT

Catalytic compositions of oxides of titanium, vanadium and tin that are suitable for the production of phthalic anhydride by oxidizing o-xylene and/or naphthalene in the gas phase. The catalysts exhibit excellent activity and selectivity. The catalyst contains 2 to 15 percent by weight (calculated as V 2 O 5 ) of vanadium, 1 to 15 percent weight (calculated as SnO 2 ) of tin and 70 to 97 percent by weight (calculated as TiO 2 ) of titanium. In a preferred embodiment the catalyst also contains up to 5 percent by weight (calculated as M 2 O) of at least one alkali metal, preferably lithium, potassium or rubidium, and more preferably cesium. In an even more preferred embodiment, cesium is present in an amount of from 0.01 to 2 percent by weight (calculated as Cs 2 O).

This application is a 371 national stage application of International(PCT) Application No. PCT/EP03/006494, filed on Jun. 19, 2003, that haspriority benefit of Italian Patent Application No. MI2002A001358, filedon Jun. 19, 2002.

The present invention relates to a catalyst for the selective oxidationof o-xylene, naphthalene, or a mixture of both to phthalic anhydride inthe gas-phase, using a gas containing molecular oxygen, preferably air.The catalyst is characterised by a high activity and a high selectivityto phthalic anhydride. It further relates to a process for thepreparation of said catalyst and a process for the production ofphthalic anhydride employing said catalyst.

Phthalic anhydride is an important chemical intermediate used for theproduction of plasticizers, alkyd resins, unsaturated polyester resinsand other commercial products.

The commercial production of phthalic anhydride is based on the gasphase oxidation of o-xylene, naphthalene, or a mixture of both. Theoxidation is performed by feeding a mixture of the hydrocarbon(s) and anoxygen containing gas, usually air, over a fixed bed of catalyst in atubular reactor tube at temperatures in the range of 300-400° C.Commercially the oxidation is carried out in multitubular fixed-bedreactors. The reaction is exothermic and the heat of reaction is removedby cooling media, usually molten salts, circulating in the shell aroundthe reactor tubes. In spite of the cooling, the reactor is notisothermal and a temperature profile with a hot spot develops along thetube from inlet to outlet. These local hot spots are undesired becausethey may damage the catalyst and favour the formation of undesired sideproducts, such as maleic anhydride, benzoic acid, carbon monoxide andcarbon dioxide. In order to limit the formation of hot spots, catalystbeds with two or three different catalysts, with graduated activity, areoften used. Their activity is lowest at the entrance of the reactants,where most of the heat is formed, and highest at the outlet of thecatalyst bed (U.S. Pat. No. 6,362,345). The use of a highly activecatalyst, especially in the lower part of the bed, i.e., near theentrance of the reactants, allows to carry out the reaction at lowertemperature, with advantages for the yield of the reaction and the lifeof the catalyst. Therefore it has been very desirable to develop highlyactive catalyst compositions which at the same time have a highselectivity for phthalic anhydride.

The catalysts used today for the production of phthalic anhydride areusually supported catalysts wherein the catalytically active material isdeposited, preferably as a coating, on an inert support in the form ofgranules or pellets, usually in the shape of spheres, cylinders orrings. The inert support in the granules or pellets of the catalyst maybe, for example, corundum, steatite, alumina, silicon carbide or anyother material having suitable chemical inertness and mechanical andthermal stability. The amount of active material deposited on the inertcarrier is usually between 1 and 15 wt. %, based on the total weight ofthe catalyst.

The active material of the catalysts currently being used in theproduction of phthalic anhydride generally comprises titanium dioxide(titania), preferably having the crystalline structure of anatase,vanadium oxide, which is spread over the titanium dioxide and chemicallyinteracts with it, and various additional components which are referredto as dopants. The dopants include elements like cesium, antimony,molybdenum, potassium, phosphorus and mixtures thereof. They are either:

-   (i) alkali metal or alkaline earth metal ions, the role of which is    claimed to tune the surface acid-basic properties of the    catalyst—alkali or alkaline earth metal ions are generally known to    increase the selectivity and to decrease the activity of the    catalysts (M. S. Wainwright, N. R. Foster, Catal. Rev. Sci. Eng.,    19 (1979) 211; V. Nikolov, D. Klissurski, A. Anastasov, Catal. Rev.    Sci. Eng., 33 (1991) 319; C. R. Dias, M. Farinha Portela, G. C.    Bond, Catal. Rev. Sci. Eng., 39 (1997) 169)—or-   (ii) transition or post-transition metal ions, the role of which is    claimed to be the control of the redox properties of vanadium ions    (M. S. Wainwright, N. R Foster, Catal. Re. Sci. Enig., 19 (1979)    211; V. Nikolov, D. Klissurski, A. Anastasov, Catal. Rev. Sci.    Etig., 33 (1991) 319; C. R. Dias, M. Farinha Portela, G. C. Bond,    Catal. Rev. Sci. Eng., 39 (1997) 169); which are considered to be    the main active sites in the reaction.

Another role of dopants can be the stabilization of the morphologicalfeatures of titanium dioxide, such as crystallinity and surface area, orthe formation of compounds with vanadium oxide having peculiarproperties.

In the oxidation of o-xylene or naphthalene, besides phthalic anhydrideseveral by products are formed, including carbon monoxide, carbondioxide, o-tolualdehyde, o-toluic acid, phthalide, maleic anhydride andbenzoic acid. In particular, these by-products are formed in hot spotswhich can develop in the reactor tubes as decribed above. Theseby-products are particularly undesired because they decrease theconversion and the yield of phthalic anhydride and some of them aredifficult to remove. In commercial application, the conversion ofo-xylene must be as high as possible and, consequently, theconcentration of unconverted o-xylene at the reactor outlet must be aslow as possible. Thus the optimal catalyst has to be as active aspossible, so to achieve a very high o-xylene conversion, and also veryselective in phthalic anhydride, leading to as low an amount as possibleof by-products.

The activity of the catalysts can be increased in different ways:

-   1) Increasing the surface area of titania, so to achieve a higher    dispersion of vanadium active sites (G. Centi, Appl. Catal., A:    general, 147 (1996) 267, and references cited therein). The main    drawback of this approach is that higher surface areas usually    result in catalysts which are less resistant towards thermal shocks    and local hot-spots, and more easily tend to exhibit    recrystallization phenomena with segregation of vanadium oxide,    responsible for a decrease of the surface area and progressive    decrease of the activity of the catalyst.-   2) Loading a higher amount of vanadium oxide while keeping the    surface area of titania constant (G. Centi, Appl. Catal., A:    general, 147 (1996) 267, and references cited therein). The    disadvantage of this approach is that it is known that an optimal    amount of vanadium exists for a given titanium dioxide surface area,    which corresponds to the formation of the so-called “monolayer” of    active species. Higher amounts of vanadium oxide are useless and    deleterious, since bulk vanadium oxide may form which does not    interact with the titanium dioxide and worsens the selectivity of    the catalyst towards phthalic anhydride.-   3) Using suitable dopants to improve the activity of the catalyst    while maintaining good performance in terms of selectivity to    phthalic anhydride (M. S. Wainwright, N. R. Foster, Catal. Rev. Sci.    Eilg., 19 (1979) 21 1; V. Nikolov, D. Klissurski, A. Anastasov,    Catal. Rev. Sci. Eng., 33 (1991) 319; C. R. Dias, M. Farinha    Portela, G. C. Bond, Catal. Rev. Sci. Eng., 39 (1997) 169). However,    dopants described so far in the literature usually have negative    effects on either the activity or the selectivity of the catalyst.

Very few examples of V/Ti/O catalysts for the oxidation of o-xylene ornaphthalene to phthalic anhydride containing tin as dopant are reported.U.S. Pat. No. 4,469,878 mentions the addition of tin as a promoter:

-   (i) tin is added as the sole promoter to a V/Ti/O catalyst, or    alternatively is added together with phosphorus;-   (ii) the amount of tin added to the catalyst composition is low:    0.1-1 wt. % of the active components, preferably 0.2-0.6 wt. %.

The performance of the catalyst is not fully satisfactory and in theexamples of U.S. Pat. No. 4,469,878 only the oxidation of naphthalene istaken into consideration.

It has been an object of the present invention to provide a catalystwhich avoids the disadvantages of the known catalysts for the productionof phthalic anhydride and has a very high activity and an excellentselectivity for the formation of phthalic anhydride, especially wheno-xylene is used as starting material. It has been another object of theinvention to provide a simple and economic process for the preparationof said catalyst starting from easily available inexpensive materials.Still another object of the invention has been to provide a process forthe production of phthalic anhydride with high conversion of thestarting hydrocarbon(s), high yield and high selectivity.

These objects have been accomplished by the catalyst of the invention,for process of the invention for its preparation and the process of theinvention for the production of phthalic anhydride.

Applicants have discovered that the activity of known catalysts can besignificantly increased by addition of comparatively large amounts oftin, without any adverse effect on the selectivity to phthalicanhydride. A further finding of the present invention is that theaddition of tin is particularly advantageous for the catalyst activityand selectivity if tin is added together with an alkali metal ion,preferably cesium.

The catalysts according to the invention comprise, based on the totalweight of the catalytically active oxidic composition (i.e., withdisregard of any inert support),. from 2to 15% by weight (calculated asV₂O₅) of vanadium and from 1 to 15% by weight (calculated as SnO₂) oftin. They further comprise from 70 to 97% by weight (calculated as TiO₂)of titanium oxide. This means that unless there are additionalcomponents (see below), titanium oxide makes up the balance to 100%.

In a preferred embodiment, the catalyst of the invention contains, basedon the catalytically active composition, up to 5% by weight (calculatedas M₂O) of at least one alkali metal. Preferably, the alkali metal islithium, potassium or rubidium, and more preferably it is cesium.

In an even more preferred embodiment, cesium is present in an amount offrom 0.01 to 2% by weight (calculated as Cs₂O), based on thecatalytically active composition.

In the finished catalyst, the tin is preferably present in the oxidationstate +IV.

Preferably, the titanium oxide which forms the basic ingredient of thecatalyst of the invention has the anatase structure and a specificsurface area of 10 to 30 m²/g, more preferably 18 to 25 m²/g.

In a particularly preferred embodiment, the catalyst of the inventioncontains, based on the catalytically active composition, from 4 to 10%by weight (calculated as V₂O₅) of vanadium oxide, from 2 to 7% by weight(calculated as SnO₂) of tin oxide and from 0.1 to 0.8% by weight(calculated as Cs₂O) of cesium oxide.

In another preferred embodiment, the catalyst of the invention containsone or more element(s) selected from the group consisting of lithium,potassium, rubidium, aluminum, zirconium, iron, nickel, cobalt,manganese, silver, copper, chromium, molybdenum, tungsten, iridium,tantalum, niobium, arsenic, antimony, cerium, phosphorus, and mixturesthereof. These elements may be present in a total amount of up to 5% byweight, based on the catalytically active composition.

Advantageously, the catalyst of the invention comprises an inert supportwhereon the catalytically active composition is deposited in an amountof from 2 to 15%, preferably 3 to 12% by weight, based on the totalweight of the catalyst including the support.

Preferably, the inert support consists of pellets or granules consistingof corundum, steatite, alumina, silicon carbide, silica, magnesiumoxide, aluminium silicate, and mixtures thereof.

The catalyst of the present invention, which can be used in commercialmultitubular reactors, may be prepared according to the followinggeneral procedure:

-   1) A mixture of the ingredients (namely: titanium oxide, vanadium    oxide and tin oxide and any additional component such as cesium) of    the catalyst's active composition, and/or of precursors which can be    converted by thermal treatment into said ingredients is prepared by    dissolving, dispersing or suspending said ingredients or precursors    in an aqueous or organic solvent, wherein the ingredients and/or    precursors are soluble or dispersible.-   2) If a supported catalyst shall be prepared, the above solution or    suspension (slurry) is coated in the form of a thin layer on an    inert support and dried or, if an unsupported catalyst is desired,    the solvent contained in the solution or slurry may simply be    evaporated and the solid residue dried and/or commninuted, if    necessary.-   3) The coated support or solid residue obtained in the preceding    step is subjected to a final thermal treatment to form the    definitive active composition.

Suitable raw materials for the production of the catalyst include:

-   TiO₂ in the form of anatase of suitable surface area, preferably    between 10 m²/g and 30 m²/g, more preferably between 18 m²/g and 25    m²/g;-   vanadium(v) oxide or, as a precursor, any vanadium compound which    can be converted by heating into vanadium(v) oxide, such as ammonium    metavanadate, vanadium chlorides, vanadium oxychloride, vanadium    acetylacetonate and vanadium alkoxides;-   tin dioxide or, as a precursor, tin compounds such as metastannic    acid, orthostannic acid, tin oxyhydrates, tin chlorides (stannic or    stannous) or tin acetate.

Preferred are tin compounds which are easily soluble or couoidallydispersible in the medium employed for catalyst preparation.

Suitable cesium compounds include cesium sulfate, cesium nitrate, cesiumchloride and any other commercial cesium salt or compound.

Suitable inert support are materials such as silica, magnesia, siliconcarbide, alumnina, aluminium silicate, magnesium silicate (steatite), orother silicates and mixtures thereof. The inert support may be ingranular form or in pellet form, usually in the form of spheres,cylinders or rings. The coating of the active compound onto the inertsupport may be accomplished by spraying the aqueous or organic solutionor slurry containing the ingredients and/or precursors on the support.This operation can be carried out in a heated drum, maintained at atemperature which is suitable for the evaporation of the solvent, forexample in the range of 50 to 250° C. The ratio between the amount ofsupport and the amount of solution or slurry, and the amount ofcomponents dissolved in the solution or suspended in the slurry, arechosen so to reach the amount of active compound which is finallydesired.

The final thermal treatment can be carried out in the heated drummentioned above, or in a separate oven, or directly in the reactor wherethe selective oxidation of o-xylene, naphthalene or mixtures of the twoto phthalic anhydride will take place. The treatment is carried out inair, or other suitable (non-reducing) atmosphere and at a temperaturewhich is typically in the range 250-450° C.

According to the invention, phthalic anhydride is prepared by oxidizinga hydrocarbon selected from the group consisting of o-xylene,naphthalene and mixtures of both in the gas phase at 340 to 400° C. withan oxygen-containing gas, preferably air, in a fixed-bed reactor in thepresence of the catalyst of the invention. Advantageously, the oxidationreaction is carried out in a multitubular fixed bed reactor. Thecatalyst granules are filled into the tubes and the feed prepared bymixing air (or oxygen or any other oxygen-containing gas) with thehydrocarbon (i.e., o-xylene and/or naphthalene) is passed over thecatalyst bed.

Preferably, the initial concentration (i.e., the concentration in thereactor feed) of the hydrocarbon is between 0.5 and 2 vol. %.

The (gauge) pressure at the reactor inlet is advantageously slightlyhigher than atmospheric, preferably between 0.35 and 0.55 bar (absolutepressure: ≈1.35-1.55 bar).

The following non-limiting examples and comparative examples describepreferred embodiments of the invention in relation to catalystsaccording to prior art:

EXAMPLE 1 (COMPARATIVE EXAMPLE)

V₂O₅ (7 wt. %) and Cs₂O (0.5 wt. %) were deposited on titania (anatase)having a surface area of 22.5 m²/g. The catalyst was prepared bydissolution of 9.0 g of (NH₄)VO₃ in 2500 ml of hot (60-80° C.) deionizedwater under stirring. Then CsNO₃ (0.069 g) was dissolved in the same,hot solution. The titania (89.5 g) was dropped in the solution and theresulting slurry was loaded in a rotary evaporator to evaporate thesolvent. The wet solid was recovered and thermally treated using thefollowing procedure, carried out in static air: Heating from roomtemperature to 150° C. at a heating rate of 10 K/min; isothermal step at150° C. for 3 h; then further heating (10 K/min) till a temperature of450° C. was reached. Final isothermal step at 450° C. for 5 h, and thencooling.

EXAMPLE 2

The same procedure as described in Comparative Example 1 was used,except for the addition of 6.65 g of an aqueous tin oxyhydrate sol(Nyacol Co., grade SN15CG) having a tin content equivalent to 15 wt. %SnO₂ and a pH of 10.0. The tin content (as SnO₂) in the fmal catalystwas 1.0 wt.% with respect to the sum of SnO₂, V₂O₅ and TiO₂.

EXAMPLES 3-6

The same procedure as described in Example 2 was used, except for theaddition of 20 g, 26.7 g, 33.3 g, and 46.7 g of the SnO₂ sol. The tincontent (as SnO2) in the final catalysts was 3.0 wt. %, 4.0 wt. %, 5.0wt. %, and 7.0 wt. %, respectively.

EXAMPLE 7 (COMPARATIVE EXAMPLE)

The same procedure as described in Comparative Example 1 was used,leading to a catalyst with the same composition, except that the surfacearea of the titania was 18 m²/g.

EXAMPLE 8

The same procedure as described in Example 4, leading to the same finalcatalyst composition (i.e., 4.0 wt. % SnO₂), was used, except that thesurface area of the was 18 m²/g and SnCl₄ was used as the Sn compound.

EXAMPLE 9 (COMPARATIVE EXAMPLE)

The same procedure as described in Comparative Example 1, leading to acatalyst with the same composition, was used, except that the surfacearea of the TiO₂ (anatase) was 34 m²/g.

EXAMPLE 10

The same procedure as described in Example 4, leading to the same finalcatalyst composition, was used, except that the surface area of the TiO₂(anatase) was 34 m²/g and the Sn compound was SnCl₄.

Catalytic Tests

The procedure for catalytic testing was the following: An uprighttubular reactor made from stainless steel, with a diameter of 1.25 cmand 25 cm long was first loaded with 11.1 g of inert material(α-alumina, 30-60 mesh particles) and then with 0.23 g of catalyst mixedwith 1.0 g of the inert material. The catalyst was shaped in granuleshaving a diameter between 30 and 60 mesh. The feed consisted of o-xylenevapour (1 vol. %) in air, and the flow rate was such to have a residencetime, measured at ambient conditions, of 0.3 s. The pressure wasatmospheric.

The results obtained are summarized in Table 1. The selectivities to andyields of phthalic anhydride (PA) are given in mole %. It is evidentfrom these results that:

-   (i) the addition of tin in the range examined (1-7 wt. % SnO₂)    results in increased activity,-   (ii) the highest increase of activity is achieved when the amount of    Sn is between about 3 and 5 wt. % SnO₂,-   (iii) that amount of Sn, and specifically an amount corresponding to    3-4 wt. % of SnO₂ also gives a higher maximum yield of PA as    compared to a catalyst without tin.

TABLE 1 Example No. T [° C.] Conv. [%] Sel. PA [%] Yield PA [%] Comp. 1327 26.4 61.1 16.1 335 39.4 70.6 27.8 347 61.5 77.0 47.4 362 78.1 82.064.0 369 86.4 83.3 72.0 375 99.1 80.6 79.9  2 335 33.8 66.6 22.5 34776.1 75.7 57.6 355 93.8 75.4 70.7 362 97.4 73.8 71.9  3 315 20.5 54.111.1 325 42.9 65.8 28.2 335 84.9 78.1 66.3 345 100 81.3 81.3  4 310 21.350.4 10.7 320 36.2 68.6 24.8 330 84.6 85.9 72.7 340 99.2 85.0 84.3  5315 23.9 54.1 12.9 325 56.3 68.8 38.7 335 93.1 73.2 68.1 345 99.5 71.971.5  6 337 46.0 64.9 29.8 349 75.4 77.4 58.4 360 99.5 79.8 79.4 368 10077.0 77.0 Comp. 7 340 32.5 68.3 22.2 350 63.8 78.5 50.1 360 71.9 78.356.3 370 80.9 78.6 63.6  8 330 18.0 48.9 8.8 339 31.8 61.0 19.4 355 83.279.9 66.5 365 96.5 81.3 78.5 371 100 81.5 81.5 Comp. 9 322 23.9 59.814.3 326 46.3 71.0 32.9 334 76.5 78.9 60.4 342 89.3 80.7 72.1 352 99.780.1 79.8 10 295 4.9 32.4 1.6 310 9.6 40.7 3.9 322 24.2 56.6 13.7 33495.8 79.6 76.2 340 100 81.0 81.0

1. An oxidic catalyst for the production of phthalic anhydride byoxidizing a hydrocarbon selected from the group consisting of o-xylenenaphthalene, and mixtures thereof, which catalyst comprises, based onthe catalytically active composition, from 2 to 15% by weight(calculated as V₂O₅) of vanadium, from 1 to 15% by weight (calculated asSnO₂) of tin, from 70 to 97% by weight (calculated as TiO₂) of titaniumoxide.
 2. The catalyst of claim 1 which further contains, based on thecatalytically active composition, up to 5% by weight (calculated as M₂O)of at least one alkali metal.
 3. The catalyst of claim 2 wherein thealkali metal is cesium.
 4. The catalyst of claim 3 wherein the cesium ispresent in an amount of from 0.01 to 2% by weight (calculated as Cs₂O),based on the catalytically active composition.
 5. The catalyst of claim1 wherein the tin is present in the oxidation state +IV.
 6. The catalystof claim 1 wherein the titanium oxide has a specific surface area of 10to 30 m²/g, preferably 18 to 25 m²/g, and the anatase structure.
 7. Thecatalyst of claim 1 wherein the catalytically active compositioncontains from 4 to 10% by weight (calculated as V₂O₅) of vanadium oxide,from 2 to 7% by weight (calculated as SnO₂) of tin oxide and from 0.1 to0.8% by weight (calculated as Cs₂O) of cesium oxide.
 8. The catalyst ofclaim 1 which further contains at least one element selected from thegroup consisting of lithium, potassium, rubidium, aluminium, zirconium,iron, nickel, cobalt, manganese, silver, copper, chromium, molybdenum,tungsten, iridium, tantalum, niobium, arsenic, antimony, cerium,phosphorus and mixtures thereof in an amount of up to 5% by weight,based on the catalytically active composition.
 9. The catalyst of claim1 wherein the catalytically active composition is coated on an inertsupport in an amount of 2 to 15% by weight, preferably 3 to 12% byweight, based on the total weight of the catalyst.
 10. The catalyst ofclaim 9 wherein the inert support consists of pellets or granules of atleast one material selected from the group consisting of corundum,steatite, alumina, silicon carbide, silica, magnesium oxide, aluminiumsilicate and mixtures thereof.
 11. A process for the production ofphthalic anhydride comprising the oxidation of a hydrocarbon selectedfrom the group consisting of o-xylene, naphthalene and mixtures ofo-xylene and naphthalene in the gas phase at 340 to 400° C. with anoxygen-containing gas in a fixed-bed reactor in the presence of acatalyst according to claim
 1. 12. The process of claim 11 wherein theinitial concentration of the hydrocarbon in the gas phase is 0.5 to 2.0%by volume.
 13. The process of claim 11 wherein the gauge pressure at theentrance of the reactor is 0.35 to 0.55 bar.
 14. The catalyst of claim 4wherein the tin is present in the oxidation state +IV.
 15. The catalystof claim 5 wherein the titanium oxide has a specific surface area of 10to 30 m²/g, preferably 18 to 25 m²/g, and the anatase structure.
 16. Thecatalyst of claim 6 wherein the catalytically active compositioncontains from 4 to 10% by weight (calculated as V₂O₅) of vanadium oxide,from 2 to 7% by weight (calculated as SnO₂) of tin oxide and from 0.1 to0.8% by weight (calculated as Cs₂O) of cesium oxide.
 17. The catalyst ofclaim 7 which further contains at least one element selected from thegroup consisting of lithium, potassium, rubidium, aluminium, zirconium,iron, nickel, cobalt, manganese, silver, copper, chromium, molybdenum,tungsten, iridium, tantalum, niobium, arsenic, antimony, cerium,phosphorus and mixtures thereof in an amount of up to 5% by weight,based on the catalytically active composition.
 18. The catalyst of claim8 wherein the catalytically active composition is coated on an inertsupport in an amount of 2 to 15% by weight, preferably 3 to 12% byweight, based on the total weight of the catalyst.
 19. A process for theproduction of phthalic anhydride comprising the oxidation of ahydrocarbon selected from the group consisting of o-xylene, naphthaleneand mixtures of o-xylene and naphthalene in the gas phase at 340 to 400°C. with an oxygen-containing gas in a fixed-bed reactor in the presenceof a catalyst according to claim
 10. 20. The process of claim 12 whereinthe gauge pressure at the entrance of the reactor is 0.35 to 0.55 bar.